The striking feature of the church in Sardis is that no defects are highlighted. The sins that marked Pergamum and Thyatira are not mentioned, nor is the lack of love that marked Ephesus. Yet although those churches were rebuked for those sins, they were also commended for what they did as churches. Sardis was not commended as a church for any activity. There is no mention of the Nicolaitans or other false teachers that they resisted. They were not experiencing persecution either, which suggests that neither the state or the powers of darkness regarded them as a threat.
Obviously, other churches had a very high opinion of the church in Sardis. Since Jesus does not list the practices of the church, we do not know what it was that caused other congregations to admire. The problem with Sardis seems to have been their inability to complete what they commenced to do. They did not have spiritual energy because they had grieved the Spirit. They are described as moribund and asleep.
Therefore they are commanded to obtain strength, otherwise even what they still have will die. Where would they get strength from? They would find it when they utilised what they had already received and heard about.
What had they received and heard in the past? No doubt, there is a reference here to the gospel and other instructions that the church would have received. Yet it is unlikely that they would have departed from them because other churches would have noticed such a relapse and not regarded them with such enthusiasm. There was something else missing that they once had, and I suspect that the reference is to the workings of the Holy Spirit. They had become a church without power because they had grieved the Spirit and he was no longer empowering them to serve the Lord. We can see now why the Saviour introduced himself to the church by reminding them that he could give the Spirit to them.
What would happen if they ignored his warning? If they did not repent, Jesus states that he will come suddenly and sort out the situation. The implication is that Jesus prefers the church to go out of existence rather than remain in a sleeping state.